Alan Scott was the first to inject botulinum toxin into the extraocular muscles for strabismus treatment. Over 1100 patients have been treated by 63 investigators. The technique has major advantages over surgery for many forms of strabismus, and the therapy appears to be safe and useful. The toxin produces temporary paralysis of the injected muscle lasting for 2-3 months. Ocular alignment is improved in most cases. The mechanism by which botulinum toxin produces temporary paralysis is understood; the mechanism that produces subsequent realignment has not been thoroughly investigated. Why does realignment continue after the paralytic effect of the toxin wears off? We will address this question by using new and innovative electronic instrumentation to quantitatively measure both temporary and prolonged changes in the passive stiffness, active force generation, and viscous friction parameters of the agonist and antagonist muscles of two rhesus monkeys during and following botulinum treatment. One eye will be injected with toxin; the other will serve as a control. -A new optoelectronic probe will be employed to accurately measure length-tension curves of the agonist and antagonist to determine changes in the passive stiffness parameters of the injected and control eyes before, during, and following paralysis produced by the toxin. -A precision computer-controlled dual-coil scleral search coil system will be used to measure changes in the active force generation parameters of the agonist and antagonist muscles by comparing saccadic trajectories of the treated and control eyes before, during, and following botulinum treatment. -A rotation/release forceps procedure will be used with a scleral search coil system to measure changes in the viscous friction parameters of the agonist and antagonist muscles of the treated and control eyes over the course of the treatment. This time-series data can be used to evaluate various models of the extraocular muscle system that have been proposed in the literature in light of the parameter changes brought about by the injection of the botulinum toxin. We plan to develop an approach in this pilot study that will lead to a definitive investigation of the mechanism that causes the realignment achieved by this promising new form of strabismus therapy. Patients would benefit from this improved knowledge in terms of more predictable results, improved dosages and fewer injections.